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C# 9 pattern matching

April 6, 2021
Tom Deseyn
Related topics:
Linux.NET

Share:

    The previous article in our C# 9 series looked at top-level programs and target-typed expressions. In this article, we’ll cover new features for pattern matching. You can find an overview of the syntax offered by previous versions of C# in C# 8 pattern matching.

    Type patterns

    When checking against a type, previous versions of C# required you to include a variable name (or a _ discard). This is no longer required with C# 9:

    // is pattern with Type
    if (input is Person)
    ...
     
    // case pattern with Type
    switch (input)
    {
      case Person:
        ...  
     
    // is pattern with tuple Type
    if (input is (int, string))
    ...
    

    Combining patterns

    With the is expression in earlier versions of C#, you could already combine patterns using regular logical operators:

    if (person is Student || person is Teacher)
    ...
    

    However, this doesn’t work for switch expressions and switch case labels. C# 9 adds support for combining patterns using the and and or keywords, which works for both if and switch:

    if (person is Student or Teacher)
    ...
     
    decimal discount = person switch
    {
       Student or Teacher => 0.1m,
       _ => 0
    };
     
    switch (person)
    {
       case Student or Teacher:
          ...
    

    The and patterns have higher precedence than the or patterns. You can add parentheses to change or clarify the precedence.

    Inverting patterns

    With C# 9, you can invert patterns using the not keyword:

    if (person is not Student)
    ...
    
    switch (person)
    {
      case not Student:
        ...
    

    An interesting case is the is not null pattern. This will check whether the reference is not null. Using != null may check something different when the type overloads the != operator.

    if (person is not null)
    ...
    

    Relational patterns

    Relational patterns allow you to compare an expression to a constant numeric value:

    decimal discount = age switch
    {
       <= 2 => 1,
       < 6  => 0.5m,
       < 10 => 0.2m,
       _    => 0
    };
    

    Patterns within patterns

    Patterns can also contain other patterns. This nesting lets you express complex conditions in a concise and readable way. The following example combines several types of patterns:

    if (person is Student { Age : >20 and <30 })
    ...
    

    Conclusion

    In this article, we looked at the new pattern matching features in C# 9. The additions allow you to express more complex conditions with a clear, concise syntax.

    In the next article, we'll explore new features for methods and functions in C# 9.

    C# 9 can be used with the .NET 5 SDK, which is available on Red Hat Enterprise Linux and Red Hat OpenShift, on Fedora, and from Microsoft for Windows, macOS, and other Linux distributions.

    Last updated: April 21, 2021

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